1. Field of the Invention
This invention relates to touch position sensitive optical waveguides. One embodiment of this invention relates to touch position sensitive waveguides in the form of screens that can be used to provide user generated feedback in response to information displayed on a synchronized light source such as a cathode ray tube (CRT).
2. Description of the Prior Art
In certain applications of computers a user gains access to information by selecting options from a menu displayed on a CRT. One method for making such a selection is by entering on a typewriter keyboard a letter or number from the menu that corresponds to the desired selection. The user may be required to make selections from a series of menus before being able to view the desired information. This operation is tedious and requires considerable time, particularly if the user is not familiar with the keyboard. The process could be considerably simplified if the user were able to indicate a selection by touching the location on the CRT screen where the desired selection appears.
U.S. Pat. No. 4,484,179, which issued to Leonard R. Kasday on Nov. 20, 1984 discloses various types of touch position sensitive optical waveguides in the form of screens that provide user generated feedback in response to information appearing on a CRT. FIGS. 8 and 9 of this patent depict a flexible screen that is normally separated by a narrow air gap from the face of the CRT. A user desiring to make a selection depresses a point on the surface of the screen by applying finger pressure, thereby bringing the corresponding point on the opposite surface of the screen into contact with the face of the CRT. When this occurs, those light rays which enter the screen and contact the user's finger are diverted from their normal path and are transmitted by internal reflection to the perimeter of the waveguide where they are detected by a number of photodiodes. The electrical signals generated by the photodiodes are analyzed electronically to determine the location at which the screen was depressed.
To function effectivley, touch position sensitive screens of the type described in the Kasday patent must be transparent enough for the user to view the information being displayed on the CRT. The screen must also transmit an amount of internally reflected light sufficient to activate the aforementioned photodiodes. In addition the screen should be capable of being deflected sufficiently to activate the photodiodes under light to moderate finger pressure and should recover its initial contour relatively rapidly once the deforming pressure is removed. Kasday teaches fabricating a flexible embodiment of a touch position sensitive screen from a transparent silicone rubber. For one reason or another many types of optically transparent materials classified "silicone rubber" are unsatisfactory for this application.
Silicone rubbers are typically polyorganosiloxanes, such as polydimethylsiloxanes, that have been cured, for example by heating them in the presence of an organic peroxide. Other methods for curing polyorganosiloxanes at ambient and elevated temperatures are known to those skilled in this art.
Cured rubbers prepared from polydiorganosiloxanes are typically relatively weak and must be combined with reinforcing agents such as finely divided silica or an organosiloxane resin to achieve the level of tensile and recovery properties required for an optical waveguide. Optically transparent silicone rubber containing these fillers are disclosed in the following U.S. Pat. Nos. 3,341,490, which issued on Sept. 12, 1967 to Burdick et al.; 3,436,366, which issued to Modic on Apr. 1, 1969; 3,915,924, which issued to Wright on Oct. 28, 1975; and Canadian patent No. 565,247, which issued to Daudt on Oct. 28, 1958. None of these patents disclose pressure sensitive optical waveguides or the criteria to be used in selecting organosiloxane reactants and fillers appropriate for these devices from the broad classes of materials described in the patents. Without a knowledge of the invention defined hereinafter one cannot use the teaching of these patents to prepare optically transparent elastomers exhibiting the properties such as compressibility and resiliency required for a touch position sensitive screen or other optical waveguide device.
One disadvantage of silica-filled compositions is that they may not transmit sufficient light of the wavelength required to activate the photodiodes located at the perimeter of the screens described in the aforementioned Kasday patent.
Liquid curable polyorganosiloxane compositions that include vinyl terminated polydiorganosiloxane, a resinous organosiloxane copolymer containing dimethylvinlysiloxy, trimethylsiloxy and SiO.sub.4 /2 units and two types of organohydrogensiloxanes are disclosed in U.S. Pat. No. 4,535,141, which issued to Kroupa on Aug. 13, 1985. One of the organohydrogensiloxanes described in the Kroupa patent contains silicon-bonded hydrogen atoms only at the terminal positions of a substantially linear molecule and is present in an amount equivalent to a concentration of silicon-bonded hydrogen atoms that is at least 1.6 times the number of vinyl radicals present in the curable composition. The other organohydrogensiloxane contains an average of at least 3 silicon-bonded hydrogen atoms per molecule.
We have found the cured elastomers prepared using the compositions described in the Kroupa patent to be less than entirely satisfactory materials for touch sensitive optical waveguides because the cured elastomer continues to harden with the passage of time and eventually becomes too hard for use as a touch sensitive optical waveguide. This phenomenon is often accompanied by the appearance of bubbles of hydrogen gas that interfere with the optical properties of the waveguide. The gradual hardening and generation of hydrogen are believed due to the reaction of the silicon-bonded hydrogen atoms present in the elastomer with moisture.
We have also found waveguides containing the preferred elastomer compositions disclosed in the aforementioned Kroupa patent to have relatively poor compressive strength. These materials may therefore not be capable of withstanding the repeated compression experienced by a touch sensitive screen of the type described in the aforementioned Kasday patent.
Another shortcoming of many prior art polyorganosiloxane elastomers and gels that would otherwise be suitable for use in a touch position sensitive screen is the absence of a smooth, readily cleanable surface that is aesthetically attractive to potential users of the device. Others, like the elastomer described in the aforementioned Kroupa patent, would probably not be sufficiently "tough" to withstand repeated deformation without fracturing, tearing or puncturing.
It is known to modify the surface properties of elastomeric polyorganosiloxanes, particularly their tactile aestheics and tendency to attract dirt and other impurities present in the atmosphere, by coating the elastomer with an adherent material that forms a smooth, readily cleanable layer. A shortcoming of many such materials is that they adversely affect those properties that make the uncoated elastomer suitable for use as a touch position sensitive optical waveguide.
U.S. Pat. No. 4,472,470, issued to Modic on Sept. 18, 1984, is directed toward dirt resistant coatings exhibitng hardness values of at least about 70 on the Shore A durometer scale. These would be too hard for use as the elastomer position of a touch-sensitive waveguide.
An objective of this invention is to provide touch position sensitive screens and other optical waveguides suitable for providing user generated feedback in conjunction with a synchronized light source such as a cathode ray tube. An additional objective of this invention is to provide polyorganosiloxane elastomers and coating materials that can be incorporated into touch position sensitive optical waveguides.